Historically, tissue making has relied on creping technology to provide a paper sheet with adequate softness and bulk. Recently, new methods have been developed for uncreped tissue manufacture with noncompressive drying methods, especially through-air drying, to achieve soft, high bulk, wet resilient structures with novel properties. For practical reasons, these methods utilize woven papermaking fabrics to provide the three-dimensional structure required in uncreped sheets if they are to have excellent mechanical properties such as high bulk, high stretch in the cross direction, and high compressive wet resiliency.
Unfortunately, woven fabrics are limited in terms of height differentials and patterns that can be achieved. There are physical constraints on what can be produced on a loom, and there are further constraints on the runnability of anything so produced. While high surface depth (characteristic peak to valley depth) may be desired in many cases in order to impart bulk, stretch, and texture to a paper web, only a narrow range of surface depths can be achieved practically in existing papermaking fabrics. Further, the surface topography of woven papermaking structures are inherently characterized by precipitous peaks and valleys with step changes in height that are typically some multiple of a filament diameter. Typically, the surface has a series of warps or chutes elevated relative to other filaments, with multiple interstices between the filaments. A probe passing along such a surface will encounter a series of sudden jumps up and down. A papermaking web deformed against such a surface becomes smoothed by the physics of paper deformation, but if the underlying fabric surface is given a high degree of surface depth, the large, precipitous peaks and valleys in the fabric can result in sharp structures in the paper web we can be perceived as grits or abrasive elements by humans using the product, especially if the sheet remains uncreped. Much more desirable would be a substrate for forming paper that could have a high degree of surface depth without precipitous peaks and valleys, but rather less abrupt structures offering more pillow-like topography against which the paper web could be deformed.
A further problem with typical woven structures for papermaking is that the filaments and the surface structure itself are largely incompressible. As a result, highly textured 3-D structures are problematic in operations where one surface contacts another, as in a pressing event or a sheet transfer between two fabrics, because most of the load, shear stress, or friction during the event is borne by a small portion of the web resting on or near the highest filaments, which can result in breaking of the web near the high spots of the substrate or other forms of damage to the web and even to the underlying substrate. In some cases, it would be desirable if the highest elements in a 3-D substrate were deformable to allow the 3-D substrate to perform better in a nip or sheet transfer point such that the integrity of the web is better maintained or the distribution of stress is more uniform as the substrate deforms. This is particularly important when the transfer or pressing event involves a first textured substrate such as a papermaking fabric and a second textured substrate such as a fabric or patterned roll, for damage to the sheet and the textured substrates can occur at contact points involving relatively high spots from both substrates unless one or both such substrates can deform to allow more uniform load or stress distributions to be established.
The use of nonwoven substrates in the formation or drying of paper is known to a limited degree, for monoplanar films and membranes have been taught for the production of tissue. In tissue making, these structures typically offer flat, planar regions for imprinting a web during a compression step in order to provide a network of densified regions surrounding undensified regions, with the densified regions providing strength and the undensified regions providing softness and absorbency. Such structures and processes lack the contoured, non-planar three-dimensionality most desirable for textured and noncompressively dried materials and, due to the lack of a non-monoplanar, 3-D wet molding surface, are incapable of providing the high bulk levels of the present invention. Such processes also result in a sheet with regions of high density and regions of low density, unlike the structures of substantially uniform density provided in the noncompressive drying method of the present invention. Further, substantially planar films are inherently limited in their ability to impart three-dimensional structures to a sheet.
Therefore, it would be desirable to provide a method for improving the degree of wet molding and surface depth that can be achieved in a soft, noncompressively dried tissue.